Grate clearing and ash removal system for gasification furnace

ABSTRACT

A relatively level fuel grate surface angle cooperates with multiple feed screws that press solid fuel material into and across the grate surface to continually clear off the grate. Air spaces between each pair of adjacent grate elements for under fire air each have a ninety degree deflector flange at the top of the air space extending from one grate element across the air space and up and over an adjacent grate element to deflect under fire airflow across the adjacent grate element to act as a grate sweeper. The smooth curved surface of the deflector flange between grate elements facilitates movement of the fuel and ash material and prevents from one grate element to the next and prevents clogging of the airflow space.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to biomass fuel gasification chambers and particularly to a grate clearing and ash removal system and method for a gasification furnace; the system comprises a relatively level grate surface angle, multiple feed screws which press fuel material into and across the level grate surface, and 90 degree deflection of the under fire air, up through the grate bar spacing, and against a deflector changing the air flow to blow across the adjacent grate bar and act as a grate sweeper. Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98

The basic gasification furnace works by providing an automatic feed of wood chips or other solid fuel onto a sloped grate with controlled air coming under the grate to keep the wood chips smoldering like charcoal to give off a gas into the gasification chamber. Then the gas is forced through a horizontal blast tube with air forced into the tube at an angle to create turbulence in the tube. The gas ignites in the horizontal tube to produce an extremely hot and strong jet blast of flame out the other end of the tube in a second chamber (which may be a boiler for heat or hot water) to apply heat to any system desired, such as a heating system, hot water heating system, evaporation system (like maple sugaring), or any other system requiring heat.

Grate clearing and ash removal is the Achilles heel of solid fuel combustion. Natural mineral contents of fuels and tramp mineral materials accreted to the fuel tend to cause major problems maintaining good air flow, and therefore good quality combustion. Steep angled grates attempt to use gravity to move the solid fuel and ash down the grate, but the gravity alone is not sufficient to keep the grate clean. Steepness of the fuel grate in a gravity feed has to be in excess of the angle of repose of the material, which for ash, which is fairly dense is fairly steep resulting in the feed screws pushing out over the top of the fuel pile, and not moving the material successfully.

The consequence is the build up and melting of ash material on the grate, causing poor fuel utilization and decreasing system output, which requires the furnace to be shut down for manual grate maintenance, or have an elaborate moving or automated grate agitation approach to minimize the grate build up and ash fusion. These “systems” are high cost and have high maintenance components which often breakdown, clog up, or ultimately fail to successfully stop the ash build up problem without frequent shut down and manual maintenance. The feed grate typically has fixed and moved grate bar rows arranged in a step-like manner, in order to achieve especially effective rearrangement of the combustible material and an optimum stoking effect. Furthermore, such a grate can work at a variable transport speed in the individual grate sections, in order to adapt the dwell time of the combustible material in the various zones of the primary region of the furnace space to the respective requirements or to the qualities of the combustible material. Examples of stepped grate bars having air passages to allow an underblast from below to supply to the combustible material with the quantity of combustion air can be found in U.S. Pat. Nos. 6,655,304; 6,497,187; 6,422,161; 6,024,031; 5,694,868; 5,673,636; 5,634,412; 5,575,642; 5,549,471; 5,245,983; 5,096,146; 4,955,296; 4,719,900; 4,676,176; 4,598,651; 4,563,959; 4,528,917; and 4,479,441.

U.S. Patent Application #20050229824, published Oct. 20, 2005 by Lefcort, discloses a two-stage wet waste gasifier and burner comprising a first-stage combustion chamber which has a symmetrical grate arrangement. The grate comprises spaced individual upright air supply tubes with flat horizontal upper surfaces swept by ash-removal plates. The height and width of the air supply tubes are selected to provide an upper surface profile approximating the natural angle of repose of the waste. Twin waste feed distribution augers span the chamber and rotate in mating troughs. The axis of rotation of each auger is sloped so that the discharge end of the auger is higher than the feed inlet end, and the trough sidewalls are inclined downwardly from the inlet end to the discharge end; these attributes promote even discharge of waste across the span of the chamber. Vertical walls of the air supply tubes are provided with air ports for supplying combustion air; the vertical orientation of the apertures and a degree of shielding of the upper surfaces of the air supply tubes inhibit clogging of the air ports. Drivers for the ash-removal plates comprise pendular arms each pivoting about an upper pivot point lying substantially in the vertical plane of symmetry, each arm being pivotally connected directly or indirectly to the ash removers. A burner of the foregoing type may be combined with a dryer for drying a portion of the waste and with a mixer that mixes dried waste with raw wet waste and feeds the mixed waste to the burner. The mixed waste can be efficiently burned yet can be efficiently conveyed. Such combination is particularly suitable for combustion of sewage sludge. The invention is provided with a control unit which regulates the pile height, fuel feed augers, thermal sensors, and ash removal.

U.S. Pat. No. 5,279,234, issued Jan. 18, 1994 to Bender, is for a controlled clean-emission biomass gasification heating system/method. A biomass fuel gasification chamber, blast tube, and heat exchange chamber are interconnected horizontally and subjected to negative drawing pressure by a large variable speed chimney fan. An auger with an air lock feeds biomass fuel automatically into the gasification chamber. Fuel is moved across the gasification chamber on a partially serrated sloping grate. Three stages of fuel activity are created: anaerobic heating for pyrolysis, combustion, and incandescent charcoal oxidation for gasification. A variable speed fan, variable flue, and directional air duct and baffles control the stages with underfire air. A programmed auger in an airtight chamber removes ash automatically. In large systems a hydraulic moving wedge floor assists the fuel feeding auger and a moving sloping grate moves the fuel. A fan and long preheating duct with baffles and fins inside the gasification chamber preheat and direct air into a blast tube leading from the gasification chamber. Openings from the preheating tube angled both longitudinally and transversely into the blast tube create turbulence in the blast tube directed away from the gasification chamber. Preheated directed air flow and the negative pressure of the chimney fan draw gases from the gasification chamber into the blast tube, crack the gases, and shoot a fire blast into the heat exchange chamber. The fire blast heats an external system. Particulates are removed producing a clean-emission exhaust gas. Temperature and air quality sensors in the chimney provide feedback signals to various system controls to maintain optimum operating conditions.

U.S. Pat. No. 6,981,455, issued Jan. 3, 2006 to Lefcort, provides a two-stage wet waste gasifier and burner comprising a first-stage combustion chamber which has a symmetrical grate arrangement. The grate comprises spaced individual upright air supply tubes with flat horizontal upper surfaces swept by ash-removal plates. The height and width of the air supply tubes are selected to provide an upper surface profile approximating the natural angle of repose of the waste. Twin waste feed distribution augers span the chamber and rotate in mating troughs. The axis of rotation of each auger is sloped so that the discharge end of the auger is higher than the feed inlet end, and the trough sidewalls are inclined downwardly from the inlet end to the discharge end; these attributes promote even discharge of waste across the span of the chamber. Vertical walls of the air supply tubes are provided with air ports for supplying combustion air; the vertical orientation of the apertures and a degree of shielding of the upper surfaces of the air supply tubes inhibit clogging of the air ports. Drivers for the ash-removal plates comprise pendular arms each pivoting about an upper pivot point lying substantially in the vertical plane of symmetry, each arm being pivotally connected directly or indirectly to the ash removers. A burner of the foregoing type may be combined with a dryer for drying a portion of the waste and with a mixer that mixes dried waste with raw wet waste and feeds the mixed waste to the burner. The mixed waste can be efficiently burned yet can be efficiently conveyed. Such combination is particularly suitable for combustion of sewage sludge. The invention is provided with a control unit which regulates the pile height, fuel feed augers, thermal sensors, and ash removal.

U.S. Pat. No. 6,485,296, issued Nov. 26, 2002 to Bender, indicates a variable moisture biomass gasification heating system and method. A sloping fuel grate allows primary air to flow up into a thick biomass fuel bed in a gasifier which produces combustible gases. Fuel feed screws keep the fuel bed at a nearly constant height in response to a level indicator. A burner above the fuel bed admits swirling secondary air and combustible gases, burning the gases to produce a flame to heat a boiler or other heat applying device. A donut shaped collar in the burner prevents gases from leaking back into the burner. A programmable logic controller maintains the system at a desired optimum level of operation based on signals from oxygen, moisture, pressure, temperature and fuel bed height sensors. The controller sends signals to controls for primary air valves and dampers, fuel feeder, and an induced draft fan and optional combined primary and secondary air fan controlling the pressure and flow of air and gases for the system. System feedback controls via the programmable logic controller enable constantly managed gasification and combustion kept at an even rate and the process being clean so that the system needs no down time for cleaning. Many elements of the system are monitored for feedback control, including: moisture content of the fuel actually being in the process, fuel bed height, pressure below the fuel bed grate and above the fuel bed in the gasifier, boiler temperature and pressure, and exhaust stack gas moisture and oxygen content. The programmable logic controller (PLC) has a modern for remote access control of the system. The PLC controls fuel feed rate, primary air feed to the gasifier and secondary air feed to the burner through an air valve and damper, recycled combustion gases to the gasifier directed at the ash auger zone, timing of ash auger operation, and speed of the induced draft fan.

U.S. Pat. No. 6,155,184, issued Dec. 5, 2000 to Stiefel, shows a process for incinerating solids on a water-cooled thrust combustion grate, and a grate plate and grate for accomplishing the process wherein primary air supplied to the combustion bed through the thrust combustion grate is deflected after exiting from a surface of the thrust combustion grate by deflector elements mounted on the surface of the thrust combustion grate. The grate required for this purpose has grate plates made from a permeable hollow element with connection pieces for supplying and draining cooling water, with primary air supply ducts that run through the grate plate from a bottom to a top. Deflector elements against which the primary air exiting the outlet is intended to impact, are disposed over openings of the primary air supply ducts.

U.S. Pat. No. 4,848,249, issued Jul. 18, 1989 to LePori, claims a system and process for conversion of biomass into usable energy. The system converts unconditioned biomass, such as cotton gin trash, into usable energy. This is accomplished by gasifying the biomass, removing the particulate char from the combustible gas using cyclonic separators, burning the gas, and using the heat to generate steam. Two stage combustion helps minimize NO.sub.x formation. The system has a horizontal auger to carry biomass feed from the hopper to the gasifying chamber. In the gasifier, inert bed particles are maintained in a fluidized state by the flow of air entering through the plenum.

Three U.S. Patents, No. 4,531,462 issued Jul. 30, 1985; No. 4,378,208 issued Mar. 29, 1983 and No. 4,334,484 issued Jun. 15, 1982 to Payne, describe a biomass gasifier combustor which operates by gasification and combustion of the biomass to produce a clean effluent gas which can be used directly for grain drying or other applications where thermal energy is required. This biomass gasifier combustor burns crop residue clean enough so that the combustion gases can be used directly for grain drying without the need for a heat exchanger to isolate the combustion gases from the drying air. The biomass gasifier combustor includes a screw feeder tube having a screw feeder disposed therein. The screw feeder forces the biomass into a first combustion chamber. Primary combustion of the biomass produces a first combustion gas. A venturi gas pump creates a negative pressure region in the gasifier, drawing the first combustion gas into a second combustion chamber. A secondary combustion takes place, completely oxidizing the organics in the primary combustion gas and producing a clean exhaust gas which can be used directly for grain drying purposes. An improved first chamber includes a manifold section for preventing the biomass from escaping into the secondary combustion chamber, and a variable height grate for allowing the ash product to fall through the holes in the variable height grate. A damper may be provided at the air inlets to control the flow rates or primary and secondary air. A damper may be placed on the exhaust eductor or venturi pump for regulating the thermal output of the system. The level of biomass in the first combustion chamber may also be monitored and automatically controlled.

U.S. Pat. No. 4,470,358, issued Sep. 11, 1984 to Prochnow, indicates a continuous solid fuel-bed degasification burner apparatus. In the combustion chamber the biomass to be degasified is placed via a conveyor screw on to a trough. In the degasification chamber there prevails a high temperature due to the vicinity of the flame chamber, so that the biomass is being degasified. The escaping gasses are burned in the flame chamber. Between the wall of the degasification chamber and an outer casing there is an air chamber in which the fresh air introduced from the outside is preheated and thereafter is guided partly through the trough and partly to another section of the flame chamber.

What is needed is a three component approach to grate clearing, utilizing only non-mechanical or existing components to achieve the grate clearing to enable longer run times, longer term combustion efficiency and load maintenance, less frequent shut down for grate maintenance, less ash fusion and clinkering, and less mechanical maintenance requirements.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an integrated three component approach to grate clearing, utilizing only a non-mechanical grate structure and existing components to achieve the grate clearing, wherein the success of the combination of these factors leads to longer run times, longer term combustion efficiency and load maintenance, less frequent shut down for grate maintenance, less ash fusion and clinkering, and less mechanical maintenance requirements.

In brief, multiple feed screws pushing the fuel against and across the grate to maintain a continually scraping action of the solid fuel on the grate, to provide a relatively level grate with only a slight downward angle into the gasification chamber to assist the feed screws in pushing the entire fuel pile including the bottom of the fuel pile across the grate to move the fuel into the smoldering gasification area and push the ashes into the augur in the ash removal bin, and to provide regularly spaced air spaces across the width of the grate to receive upward air flow and orthogonal air directing flanges above each air space and over the top of the adjacent grate element to direct the airflow over each grate element parallel to the grate element in a continual cleaning action and to provide a smooth transition of the fuel and ash material from one grate component to the next and prevent fuel and ash from falling into the air spaces, thereby utilizing a three component approach to grate clearing, utilizing only a non-mechanical grate structure and existing components to achieve the grate clearing.

The present invention provides a three component approach to grate clearing, utilizing only non-mechanical components or existing components to achieve the grate clearing.

The combination of these three factors allows for more successful grate clearing and maintenance. It eliminates the need for to frequent shut down, and/or mechanical systems to maintain the grate surface area. This allows for longer run times, better fuel utilization, longer term combustion efficiency and load maintenance, less ash fusion and clinkering, less frequent shut down for grate maintenance, and increased quality combustion, and therefore improved air quality.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other details of my invention will be described in connection with the accompanying drawings, which are furnished only by way of illustration and not in limitation of the invention, and in which drawings:

FIG. 1 is a partial cross-sectional view of the fuel grate of the present invention showing the interconnected grate bars;

FIG. 2 is a partial cross-sectional view of the gasification chamber of a biomass gasification furnace showing the fuel grate and the multiple fuel feed augers of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1 and 3, an integrated system combines fuel grate structure 10 and angle and multiple feed auger 15 fuel movement components and under-fuel air flow directing components 27 for a grate clearing and fuel movement integrated system in a biomass gasification furnace.

In FIG. 1, the multiple feed screws 15 feed biomass fuel material 25 through a biomass fuel feed opening into a gasification chamber 20 of the biomass gasification furnace.

In FIG. 2, the fuel grate 10 comprises a series of interconnected grate elements 22 aligned in a linear array having aligned top surfaces 26. The grate extends from the biomass fuel feed opening through which the multiple feed screws 15 pass into and across the gasification chamber 20 angled downwardly at a slight acute angle from the horizontal so that the feed screws 15 press the fuel material 25 into and across the aligned top surfaces 26 of the grate elements to keep the bottom of the solid fuel moving along the aligned top surfaces of the grate elements in a scraping cleaning action during gasification and to push resulting ash byproduct 25B down the grate and into an ash removal auger 24 for removal.

Each adjacent pair of grate elements 22 has an air space 28 therebetween to receive under fire airflow 23 therethrough. Each grate element 22 further comprising an airflow directing flange 27 extending from one grate element over the air space 28 and over the top surface 26 of the adjacent lower grate element parallel to the top surface 26 of the lower grate element to direct the airflow 23 over the lower grate element so that the air flow blows across the top surface 26 of the lower grate element to act as a grate sweeper to clean the top surface.

The airflow directing flange 27 comprises a smooth curved top surface transitioning smoothly into a smooth flat top surface over the lower grate element to facilitate movement of the fuel and ash material from one grate element to the next and to prevent clogging of the air space 28 therebetween.

Each of the grate elements comprises a grate bar 21 extending across the fuel grate, the grate bar 21 formed from a metal square U-shaped channel having an extending top of one leg bent over the adjacent grate bar to form the airflow deflector 27. The square U-shaped channel further comprises a high temperature refractory material 26 filling the channel to form a flat top surface aligned with the short leg of the square U-shaped channel.

In use, the present invention utilizes multiple feed screws 15 driven by a motor 18 to press the fuel material 25 from a solid fuel hopper system 16 into and across the grate 10, successfully moving the fuel material into and across the grate as the carbon is removed by gasification in the smoldering fuel 25A. It also continues to push the resulting ash byproduct 25B down the grate and into the ash removal auger 24 for removal, as shown in FIG. 2.

The second factor in combination with the others is a relatively level grate 10 surface angle. The grate of the present invention is fairly level; and therefore allows the continuous pushing of the solid fuel material 25 along the grate 10 from the pressure of the multiple feed screws 15, rather than pushing out over the top of the fuel pile, and not moving the material successfully as is common in prior art having steep grates which rely on gravity form moving the fuel and ash down the grate.

The third component is the (90 degree) deflection of the under fire airflow 23, up through the grate bar spacing 28 and against the deflector 27 changing the air flow to blow across the adjacent grate bar top surface 26 and act as a grate sweeper, rather than go straight up through the fuel pile.

It is understood that the preceding description is given merely by way of illustration and not in limitation of the invention and that various modifications may be made thereto without departing from the spirit of the invention as claimed. 

1. An integrated system of fuel grate and fuel movement components and under-fuel air flow directing components for grate clearing and fuel movement in a biomass gasification furnace, the system comprising: multiple feed screws to feed biomass fuel material through a biomass fuel feed opening into a gasification chamber of a biomass gasification furnace; a fuel grate comprising a series of interconnected grate elements aligned in a linear array having aligned top surfaces, the grate extending from the biomass fuel feed opening into and across the gasification chamber angled downwardly at a slight acute angle from the horizontal so that the feed screws press the fuel material into and across the aligned top surfaces of the grate elements to keep the bottom of the solid fuel moving along the aligned top surfaces of the grate elements in a scraping cleaning action during gasification and to push resulting ash byproduct down the grate and into an ash removal auger for removal; each adjacent pair of grate elements having an air space therebetween to receive under fire airflow therethrough, and each grate element further comprising an airflow directing flange extending from one grate element over the air space and over the top of the adjacent lower grate element parallel to the top surface of the lower grate element to direct the airflow over the lower grate element so that the air flow blows across the top surface of the lower grate element to act as a grate sweeper to clean the top surface; thereby providing an integrated system of fuel grate and fuel movement components and under-fuel, airflow directing components for grate clearing and fuel movement in the biomass gasification furnace.
 2. The system of claim 1 wherein the airflow directing flange comprises a smooth curved top surface transitioning smoothly into a smooth flat top surface over the lower grate element to facilitate movement of the fuel and ash material from one grate element to the next and to prevent clogging of the air space therebetween.
 3. The system of claim 2 wherein each of the grate elements comprises a grate bar extending across the fuel grate, the grate bar formed from a metal square U-shaped channel having an extending top of one leg bent over the adjacent grate bar to form the airflow deflector, the square U-shaped channel further comprising a high temperature refractory material filling the channel to form a flat top surface aligned with the short leg of the square U-shaped channel. 